Integration of the retrovirus genome into the chromosomes of a host cell is an essential step in the propagation of this class of virus. Although recent studies using restriction endonuclease technology have approached this area of research, several unanswered questions remain about the interactions between the host cell and virus genomes. These questions include the general mechanism of integration, the process of sarcoma virus generation, and mechanisms involved in the transcriptional control of endogenous retrovirus genomes. To provide answers to some of these questions, I have developed an indirect in situ hybridization technique which can determine the chromosomal location of non-reiterated genes in vertebrate organisms regardless of gene size or host genome complexity. Preliminary results indicate that all 4 of the non-transcribed RAV-O endogenous avian retrovirus genomes that have been studied are located on chromosome 1. This non-random distribution suggests that several of those loci have evolved from a single progenitor locus. In addition, the endogenous src gene, which appears to cause cell transformation when transcribed in large amounts, is located on a small macrochromosome 10, 11, or 12, and is thus physically separated from the non-transcribed RAV-O loci.